1. Field of the Invention
The present invention relates to a solid state imager device having an electronic shutter function and a driving method thereof.
2. Description of the Related Art
There has been proposed various methods for operating an electronic shutter in a two-dimensional imaging sensor employing a charge coupled device (CCD) or the like to electrically control an exposure time without a mechanical shutter. Recently, to use a substrate arranged on a lower side of a light receiving portion as a drain and sweep out electric charges accumulated for a non-exposure period into the substrate has been a dominant method for controlling the exposure time in the art. (See "Television society technical report" TBES' 88-6: February 1988).
FIG. 5(a) is a sectional view showing a light receiving portion and its vicinity of an example of a solid state imager device to which the present invention can be applied. FIG. 5(b) is a potential profile along line A--A' in FIG. 5(a). The solid state imager device comprises a P-type well. 2 formed on a N-type substrate 1, a N-type layer 3 of a light receiving portion and a transfer portion 5 formed on the P-type well 2, a high density P.sup.+ -type layer 4 formed on a surface side of the N-type layer 3, and an electrode 6 of polysilicon or the like formed above the transfer portion 5 as shown in FIG. 5(a). The P-type well 2 is electrically grounded, and there is provided with a voltage application circuit which selectively applies a relatively low voltage V.sub.1 of about 5 V or a sufficiently high voltage V.sub.2 of about 30 V to the N-type substrate 1 in correspondence with an external signal.
As shown in FIG. 5(b), signal charges are accumulated in the N-type layer 3 of the light receiving portion in such a state that the lower voltage V.sub.1 is applied to the N-type substrate 1, because a potential barrier is formed in the vicinity of the interface between the P-type well 2 and the N-type layer 3 (hatched portion shown in FIG. 5(b)).
On the other hand, when the higher voltage V.sub.2 is applied to the N-type substrate 1, the potential barrier is extinguished and all the signal charges accumulated in the light receiving portion of the N-type layer are swept out into the N-type substrate 1. Thus, to the control the voltage applied to the N-type substrate 1 enables a shutter operation that charges are not accumulated except for a desired period. Consequently, the exposure period for the shutter operation ranges from the point of time when the voltage applied to the N-type substrate has been changed from V.sub.2 to V.sub.1 to the point of time when the signal charges accumulated in the light receiving portion are readout in the transfer portion.
FIGS. 6(a)-(c) are timing charts of signals in the case where the above-described shutter operation is simply realized. FIG. 6(a) is a vertical synchronization timing chart in which a one cycle is one field period consisting of an effective vertical scanning period and a vertical blanking period. As shown in FIG. 6(b), a readout pulse is generated in the latter half of the vertical blanking period in order to activate the readout operation of the charges accumulated in the light receiving portion. In a shutter pulse shown in FIG. 6(c), the voltage applied to the N-type substrate 1 is predetermined to the lower voltage V.sub.1 in the effective exposure period ranging from the beginning of receiving light to the generation of the readout pulse and to the higher voltage V.sub.2 in the ineffective exposure period.
However, a voltage variation of the shutter pulse is 25 V in the method shown in FIG. 6, which is considerably high in comparison with an average image signal of 300 mVp-p. Accordingly, since the signal ground voltage of the solid state imager device varies due to such a high voltage variation, signal level variation occurs nearly in the variation point of the shutter pulse when the variation point is within an image signal period. As a result of the signal level variation, horizontal striping occurs on an output image plane due to the variation of luminance brightness as shown in FIG. 6(d).
An example of the shutter operation in which the variation point of the shutter pulse does not undesirably effect on the image is disclosed in the Japanese Unexamined Patent Publication JP-A 63-105579 (1988) and the U.S. Pat. No. 4,875,100. In the example of the shutter operation, the signal level variation in the image signal period is prevented by setting the variation point of the shutter pulse so as to be within the horizontal blanking period.
FIGS. 7(a)-(f) are timing charts of the shutter operation where the variation point of the shutter pulse is set to be within the horizontal blanking period. FIG. 7(a) is a timing chart of vertical synchronization where one cycle is considered to be one field period consisting of an effective vertical scanning period and a vertical blanking period (the same as that shown in FIG. 6(a)). A readout pulse is also generated in the latter half of the vertical blanking period in order to activate the readout operation of the charges accumulated in the light receiving portion as shown FIG. 7(b) The predetermination of the effective exposure period from the beginning of receiving light to the generation of the readout pulse is changed in correspondence with the intensity of incident light.
FIG. 7(c) is a timing chart of the shutter pulse in the case where the intensity of light incident to the light receiving portion is low and the effective exposure period from the beginning of receiving light to the generation of the readout pulse is set to be relatively long. The effective exposure period means a period when charges utilized for an image signal are accumulated. In this case, the signal level variation is not caused for an image signal period, because the rising and falling of the shutter pulse are completed within a horizontal blanking period of the horizontal scanning period (1H period) in the ineffective exposure period. The ratio of the horizontal blanking period to one horizontal scanning period amounts to about 17%, for example, in the device complying with National Television System Committee Standard.
FIG. 7(d) is a timing chart of the shutter pulse in the case where the intensity of light incident to the light receiving portion is high and the effective exposure period is set to be shorter. The signal level variation is not caused for an image signal period, because the rising and falling of the shutter pulse are also completed within the horizontal blanking period in the ineffective exposure period (the same as that shown in FIG. 7(c)).
In the conventional shutter operation described with reference to FIGS. 7(a)-(f), however, as the intensity of the incident light is enhanced, a large-quantity of charges are generated only within a short period of an effective horizontal scanning period. Consequently, the light receiving portion is flooded with the charges and the charges overflow into other light receiving portions therearound or into transfer portions, which causes a so-called blooming phenomenon.
The phenomenon will be described in detail in the following. FIG. 7(e) is a timing chart of the volume of signals accumulated in the light receiving portion in the case where the intensity of incident light is lower and the shutter operation is conducted with the shutter pulse shown in FIG. 7(c). More specifically, the signal volume increases with a gradient proportional to the intensity of the incident light in the effective exposure period ranging from the beginning of receiving light to the generation of the read out pulse, and the signal volume at the time of termination of the effective exposure period is read out as an image signal level. On the other hand, in the ineffective exposure period, the signal volume increases with the same gradient within the effective horizontal scanning period and each time the signal volume is cleared to be zero by means of the shutter pulse generated in each horizontal blanking period.
FIG. 7(f) is a timing chart of the volume of signals accumulated in the light receiving portion in the case where the intensity of incident light is higher and the shutter operation is carried out with the shutter pulse shown in FIG. 7(d). In this case, the signal volume increases with a gradient proportional to the intensity of the incident light in the effective exposure period and the signal volume at the time of the termination of the effective exposure period is read out as an image signal level. On the other hand, though the signal volume increases with the same gradient in the effective horizontal scanning period of the ineffective exposure period, the signal volume is saturated at a certain level, because most of charges generated in the light receiving portion overflow the potential barrier into the other light receiving portions therearound or transfer portions.
It is required in the electronic shutter operation to obtain a normal image by controlling the exposure period even in the case of an extremely high intensity of incident light. A period for sweeping out the charges into the substrate, however, is limited to a shorter period within a one horizontal scanning period (1H) (the shorter period is, for instance, about one fiftieth of 1H) in the shutter pulse method as shown in FIGS. 7(c) and (d), and therefore, as shown in FIG. 7(f), excess charges overflow the light receiving portion, which causes a failure of activating the shutter operation in the ease of the extremely higher intensity of incident light.